1. Field of the Invention
The present invention is used for optical communication and concerns especially, an optical amplifier used for Raman amplification.
2. Description of Related Art
In a wavelength division multiplex (WDM) optical transmission system, signal channels having a plurality of different wavelengths are simultaneously transmitted in one optical fiber. This means that in a WDM transmission system, increasing the number of wavelengths (the number of signal channels) expands the transmission capacity for each optical fiber thereby enabling greater economy in the number of optical fibers.
A WDM optical transmission system is comprised mainly of: (1) an optical transmitter installed in a transmitting terminal, (2) an optical receiver installed in a receiving terminal and (3) an optical transmission line connecting the optical transmitter and receiver. Usually, an optical transmission line is comprised of: (A) a plurality of optical amplifiers such as erbium doped optical fiber amplifiers, for amplifying WDM optical signals and (B) a plurality of transmission optical fibers connecting between the optical amplifiers.
The signal quality of signal light output from an optical transmitter deteriorates as it is transmitted through an optical transmission line. The primary factors of this deterioration are degradation through amplified spontaneous emission (ASE) and degradation due to nonlinear effects. ASE refers to a constant amount of light with random phases. Nonlinear effects include various kinds of effects, among which distortion due to self phase modulation group velocity dispersion (SPM-GVD) is the most typical one. Distortion due to SPM-GVD refers to a wave distortion caused by time-dependent nonlinear refractive index changes in the transmission fiber induced by signal light pulses. The distortion increases with the signal light power.
Signal deterioration caused by ASE has a tendency to increase as signal light power output from transmission fiber decreases. This is because the output terminal of a transmission fiber is normally connected to the input terminal of an optical amplifier which usually generates a constant amount of optical noise. Therefore, if signal power output from transmission fiber decreases, the ratio of signal power to ASE, that is to say, the signal-to-noise ratio (SNR) decreases leading to signal degradation. In other words, to reduce the effects of ASE, higher signal light power is preferable at the output of a transmission fiber.
Signal deterioration from nonlinear wave distortion caused by SPM-GVD has a tendency to increase as signal light power input into transmission fiber increases. This is because the nonlinear refractive index change that causes SPM-GVD nonlinear waveform distortion increases together with signal light power. Accordingly, to reduce the effects of nonlinear wave distortion caused by SPM-GVD, lower signal light power is preferable at the input of a transmission fiber.
Accordingly, due to these two kinds of signal deterioration, higher signal light power is preferable at the output of a transmission fiber while lower signal light power is preferable at the input of a transmission fiber. However, because the output signal light power and the input signal light power is related uniquely though the optical transmission fiber loss, both those conditions cannot be satisfied simultaneously. Therefore, signal light power is usually set to minimize the overall deterioration from the both kinds.
Even with signal light power at optimum conditions, both these deterioration cannot be completely avoided. Further, both these kinds of deterioration accumulate and worsen as signals pass through a plurality of optical amplifiers and transmission fibers. Therefore, the maximum transmission distance for an optical transmission system is usually restricted by the accumulation of deterioration through both causes.
For relaxing such restrictions, methods and systems using Raman amplification are proposed. Raman amplification refers to a phenomenon wherein amplification of signal light inside transmission fiber is achieved via the stimulated Raman effect. This occurs as pumping light, light of specific wavelengths different from the wavelengths of the signal lights, is input into the transmission fiber simultaneously with the signal light. There are three configurations for a system using Raman amplification: the xe2x80x9cbackward pumpingxe2x80x9d configuration where backward pumping light is transmitted in the opposite direction to the signal light; the xe2x80x9cforward pumpingxe2x80x9d configuration where forward pumping light is transmitted in the same direction as the signal light; and the xe2x80x9cbidirectional pumpingxe2x80x9d configuration where both forward pumping light and backward pumping light are transmitted in the same direction and the opposite direction to the signal light, respectively. By using Raman amplification of any of these three configurations, it is possible to achieve better signal quality than provided by a configuration not applying Raman amplification, if signal light power of at the input of the transmission fiber is appropriately set.
Improved signal quality through forward pumping will now be described with reference to FIG. 13. In FIG. 13 distance along the transmission fiber is plotted on the horizontal axis and average signal light power is plotted on the vertical axis. Average signal light power in transmission fiber for a system not applying Raman amplification is shown by (a) of FIG. 13. Here, if forward pumping light is injected from the input terminal of the transmission fiber, the signal light is amplified within the transmission fiber and signal power inside the transmission fiber changes as shown by (b) of FIG. 13. If, here, input signal power is lowered so that average signal light output from the transmission fiber is the same as before Raman amplification is applied, the result would be as shown by (c) of FIG. 13. This means that as it is possible to decrease input signal light power while maintaining the output signal light power from the transmission fiber constant (keeping deterioration caused by ASE constant). This means that nonlinear waveform distortion degradation from SPM-GVD can be reduced, leading to improved signal quality.
Four wave mixing (FWM) is another typical kind of nonlinear effect. FWM refers to the effect of power conversion between four lightwaves via the third order optical nonlinear effect of a transmission fiber.
This effect occurs whether the wavelengths of the four lightwaves are all the same, are all different or whether some are the same and some are not. Usually, the field of optical communication is concerned with the case where all four of the lightwaves are of different wavelengths or where two of the four lightwaves have the same wavelengths while the other 2 have different wavelengths. In the former case, if the wavelengths of the four lightwaves in ascending order of wavelength are xcex1, xcex2, xcex3, xcex4, power conversion between these four lightwaves through FWM arises when for example the following conditions are met:
xcex4=xcex1xc3x97xcex2xc3x97xcex3/(xcex1xc3x97xcex2xe2x88x92xcex2xc3x97xcex3+xcex3xc3x97xcex1) 
If xcex1=1530 nm, xcex2=1535 nm, xcex3=1580 nm for example, power conversion between the four lightwaves occurs when xcex4=1585 nm. This results in the power elements of wavelengths xcex1, xcex2, xcex3 to be mixed into the lightwave of wavelength xcex4. (If there was no wave of wavelength xcex4 originally, a new lightwave would arise). Because these four lightwaves represent different wavelength channels, mixture of signal light leads to deterioration of signal quality (For example, for wavelength channel of xcex4, power elements xcex1, xcex2, xcex3 are noise). Thus, FWM should be avoided. A means for avoiding signal quality degradation caused by FWM is to allocate signal light wavelengths avoiding the conditions of the above expression. Lowering input signal light power into the transmission fiber is also effective because the FWM increase together with the power of the four lightwaves. Information on this method can be found in H. Suzuki et al, Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication ""99 Technical Digest, ThO4 or JP 2000-330145 A for example. Further, because power conversion efficiency in FWM decreases as the absolute value of the chromatic dispersion on transmission fiber increases, the effect of FWM can be suppressed by using transmission fiber with absolute value of chromatic dispersion equal to 2 ps/nm/km or greater at the wavelength bands of signal light. For wavelength band in the 1550 nm region for example, fiber with chromatic dispersion of 2 to 10 ps/nm/km, called nonzero dispersion shifted fiber, can be utilized.
If however Raman amplification is applied for fiber having chromatic dispersion of 2 to 10 ps/nm/km at the wavelength band of the signal light, problems arise. Even though the absolute value of chromatic dispersion may be sufficiently large at the wavelength band of the signal light, it may be too small at the wavelength region of the pumping light, leading FWM to occur strongly. Accordingly, due to FWM between pumping lights, and/or FWM between pumping lights and signal lights, there is a possibility of power mixture between pumping lights and signal lights, causing distortion and a deterioration in signal quality. This problem is particularly significant when the forward pumping configuration of Raman amplification is used. At present, there are no papers clarifying the issues and providing solutions concerning FWM effects on pumping light for Raman amplification, thus the problems remain unsolved.
Against the background of these problems the object of the present invention is to provide a Raman amplifier, pumping apparatus and program therefor that enables the problem of deterioration in signal quality to be avoided by suppressing the occurrence of lightwaves from FWM in wavelength bands of signal light by appropriately setting the wavelengths or power, or both, of pumping lights used for Raman amplification.
According to the present invention, in a Raman amplifier wherein signal light is provided with gain in a Raman amplification medium through the stimulated Raman effect and which comprises a pumping means for injecting pumping light into the Raman amplification medium, the wavelengths of the lightwaves are allocated so that wavelengths of lightwaves arising from a nonlinear effect of the pumping lights generated by the pumping means or the pumping light and the signal light, or both, do not fall within the wavelength band of the signal light.
Basically, this invention is characterized in that the pumping means generates pumping lights of a plurality of wavelengths and in respect of two arbitrary pumping lights of those pumping lights of a plurality of wavelengths, the peak wavelength, xcexmin, of pumping light having the shorter wavelength and the peak wavelength, xcexmax, of pumping light having the longer wavelength are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) 
or
xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) greater than xcexlong. 
Three arbitrary pumping lights from among the a plurality of pumping lights, the peak wavelengths of which three pumping lights satisfy the expression
xcexmin less than xcexmid less than xcexmax 
are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) 
or
xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) greater than xcexlong. 
With the present invention, the wavelengths or the power, or both, of the pumping lights are allocated or set so that, lightwave power arising through a nonlinear effect of pumping lights generated by the pumping means, or the pumping light and the input light signal, or both, is, within the wavelength band of the signal light, at all points of the Raman amplification medium, 20 dB or more smaller than or one part per hundred or less of, the power of the signal light.
Thus, with the present invention, by setting the wavelength or power, or both, of pumping lights, deterioration of signal light quality due to lightwaves arising through FWM can be avoided or suppressed.
This invention is especially effective where the absolute value of chromatic dispersion in the transmission fiber is equal to or greater than 2 ps/nm/km and equal to or less than 10 ps/nm/km anywhere within the wavelength band of the signal light.
The present invention is also effective in a system wherein zero-dispersion wavelength of the transmission fiber is equal to or greater than the peak wavelength of the pumping light having the shortest peak wavelength from among the pumping lights and equal to or less than the peak wavelength of the pumping light having the longest peak wavelength from among the pumping lights. Further, this invention is especially effective when the forward pumping configuration is used.
That is to say, according to a first aspect of this invention, it is provided a Raman amplifier comprising a Raman amplification medium that provides signal light with gain though the stimulated Raman effect and a pumping means for injecting pumping lights of a plurality of wavelengths in this Raman amplification medium.
This invention is characterized by comprising a means for allocating wavelengths of the pumping lights, so that in respect of two arbitrary pumping lights among the pumping lights of a plurality of wavelengths generated by the pumping means, the peak wavelength xcexmin, of pumping light having the shorter wavelength and the peak wavelength xcexmax, of pumping light having the longer wavelength are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) 
or
xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) greater than xcexlong. 
Alternatively, this invention is characterized by comprising a means for allocating wavelengths of the pumping lights so that three arbitrary pumping lights from among the pumping lights of a plurality of wavelengths generated by the pumping means, the peak wavelengths of which three pumping lights satisfy the expression
xcexmin less than xcexmid less than xcexmax 
are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) 
or
xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) greater than xcexlong. 
Alternatively, this invention is characterized by comprising a means for setting or allocating the wavelengths or the power, or both, of the pumping lights, so that the power of lightwaves arising through a nonlinear effect of the pumping lights generated by the pumping means, or of the pumping lights and the input signal, or both, is, within the wavelength band of the signal light, at all point of the Raman amplification medium, 20 dB or more smaller than or one part per hundred or less of, the power of the signal light. This nonlinear effect could be for example, FWM.
It is preferable for the Raman amplification medium to include optical fiber, the absolute value of the chromatic dispersion of which fiber is, equal to or greater than 2 ps/nm/km and equal to or less than 10 ps/nm/km anywhere within the wavelength band of the signal light.
It is also preferable for the Raman amplification medium to include optical fiber, the zero-dispersion wavelength of which optical fiber is equal to or greater than the peak wavelength of the pumping light having the shortest peak wavelength from among the pumping lights and equal to or less than the peak wavelength of the pumping light having the longest peak wavelength from among the pumping lights.
It is preferable for the present invention to provide a forward pumping means for transmitting at least a portion of pumping light generated by the pumping means, inside the Raman amplification medium in the same direction as the signal light.
According to a second aspect of this invention, it is provided a pumping apparatus comprising a pumping means for injecting pumping lights of a plurality of wavelengths into the Raman amplification medium that provides signal light with gain though the stimulated Raman effect.
The second aspect of this invention is characterized by comprising a means for allocating wavelengths of the pumping lights so that, in respect of two arbitrary pumping lights among the pumping lights of a plurality of wavelengths generated by the pumping means, the peak wavelength xcexmin, of pumping light having the shorter wavelength and the peak wavelength xcexmax, of pumping light having the longer wavelength are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) 
or
xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) greater than xcexlong. 
Alternatively, the second aspect of this invention is characterized by comprising a means for allocating wavelengths of the pumping lights so that three arbitrary pumping lights from among the pumping lights of a plurality of wavelengths generated by the pumping means the peak wavelengths of which three pumping lights satisfy the expression
xcexmin less than xcexmid less than xcexmax 
are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) 
or
xe2x80x83xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) greater than xcexlong.
Alternatively, the second aspect of this invention is characterized by comprising a means for setting or allocating the wavelengths or the power, or both, of the pumping lights, so that the power of light arising from a nonlinear effect of the pumping lights generated by the pumping means, or of the pumping lights and the input signal, or both, is, within the wavelength band of the signal light, at all points of the Raman amplification medium, 20 dB or more smaller than or one part per hundred or less of, the power of the signal light. This nonlinear effect could be for example, FWM.
It is preferable for the second aspect of this invention to comprise a forward pumping means for transmitting at least a portion of pumping light generated by the pumping means in the same direction as the light signal inside the Raman amplification medium.
The third aspect of this invention is a program, characterized in that, installation of this program in an information processing system, provides for that information processing system, as functionality for allocating wavelengths of pumping lights applicable for a Raman amplifier comprising a Raman amplification medium that provides signal light with gain though the stimulated Raman effect and a pumping means for injecting pumping lights of a plurality of wavelengths into this Raman amplification medium, or for a pumping apparatus comprising a pumping means for injecting pumping lights of a plurality of wavelengths into a Raman amplification medium that provides signal light with gain though the stimulated Raman effect, functionality that realizes allocation of wavelengths of the pumping lights so that, in respect of two arbitrary pumping lights among the pumping lights of a plurality of wavelengths generated by the pumping means, the peak wavelength xcexmin, of pumping light having the shorter wavelength and the peak wavelength xcexmax, of pumping light having the longer wavelength are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) 
or
xcexminxc3x97xcexmax/(2xcexminxe2x88x92xcexmax) greater than xcexlong. 
Alternatively, the third aspect of this invention realizes functionality for allocating wavelengths of the pumping lights wherein three arbitrary pumping lights from among the pumping lights of a plurality of wavelengths generated by the pumping means, the peak wavelengths of which three pumping lights satisfy the expression
xcexmin less than xcexmid less than xcexmax 
are in a relationship to the shortest wavelength of the wavelength band of the signal light, xcexshort and the longest wavelength of the wavelength band of the signal light, xcexlong, expressed as
xcexshort greater than xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) 
or
xcexmaxxc3x97xcexmidxc3x97xcexmin/(xcexmidxc3x97xcexminxe2x88x92xcexmaxxc3x97xcexmid+xcexmaxxc3x97xcexmin) greater than xcexlong. 
Moreover, installing the program according to the third aspect of this invention in an information processing system, comprises that information processing system, as functionality for allocating or setting the wavelengths or power, or both, of pumping lights applicable for a Raman amplifier comprising a Raman amplification medium that provides signal light with gain though the stimulated Raman effect and a pumping means for injecting pumping lights of a plurality of wavelengths into this Raman amplification medium, or for a pumping apparatus providing a pumping means for injecting pumping lights of a plurality of wavelengths in a Raman amplification medium that provides signal light with gain though the stimulated Raman effect, functionality that realizes allocation or setting of the wavelengths or power, or both, of the pumping lights, so that the power of light arising through a nonlinear effect of pumping lights generated by the pumping means, or of pumping lights and the signal light, or both, is, within the wavelength band of the signal light, at all points of the Raman amplification medium, 20 dB or more smaller than or one part per hundred or less of, the power of the signal light. This nonlinear effect could be for example, FWM.
By recording the program of this invention onto a recording medium, the recording medium can be used to install the program of this invention in the information processing system. Alternatively, the program of this invention can be installed directly in the information processing system from a server storing the program connected to the system via a network.
Accordingly, a Raman amplifier and pumping apparatus can be realized that enable the problem of deterioration in signal quality to be avoided by means of suppressing the occurrence of lightwaves from FWM in the wavelength band of signal light by setting the wavelengths or power or both the wavelengths and power of pumping light used for Raman amplification appropriately using an information processing system like a computer.